Even mild dehydration, a loss of just 1 percent to 2 percent of body weight, can produce symptoms including weakness, dizziness, fatigue, headache, and reduction in mental and physical performance.
“The idea of drinking water is really essential to maintain normal good health,” says Dr. James Dunford, emergency room physician with UCSD Medical Center and medical director of San Diego's EMS system. “Water is needed to help preserve body functions and vital organs and maintain blood pressure. It brings nutrients to the cells and washes away the toxins. Water is the most important molecule we commonly take for granted.”
In one of my CFS books, it comments that you can tell how bad a person's CFS is by the size of their water bottle. Well, yes and no. I do drink a lot of water when I'm running a fever or when we're having triple digit temperatures, but other than that, I don't drink huge amounts of water. I usually sip at the same 20-ounce bottle all day (plus beverages with meals).
This water-bottle theory may be another of those lookalike misdiagnoses, where people think they have CFS, but what they really have is "weakness, dizziness, fatigue, headache, and reduction in mental and physical performance" from dehydration. As with every other lookalike condition, there are symptoms and test results that differentiate it from CFS.
Unfortunately, over the past 20 years, the definition of CFS has been watered down to include anyone with fatigue for any reason. Some of this was done by groups with ulterior motives (e.g., psychologists who saw a chance for profit in confusing CFS with an entirely different condition with a similar name and thus persuading the government to mandate years of psychotherapy for Disability recipients). Some of it was done by people who don't understand that there's more to Chronic Fatigue Syndrome than chronic fatigue.
As a result, there are a lot of doctors out there who apply the CFS label to people who don't have CFS, who never had CFS, who the experts would tell "you don't have CFS", who the neurological and immune system tests would prove don't have CFS.
Do yourself a favor -- if you don't have the symptoms described by the original CFS definition (such as fever, swollen lymph glands, and sore throat), hie thee to a CFS expert and get the right diagnosis. If you have something as simple as dehydration, it's much easier to get back to feeling well than if you actually have CFS.
On the other hand, while you're waiting for that appointment, do your own First Aid experiments. When I started fainting right after starting a new medication, by the time I went to the doctor, I was already able to tell him that I'd tried eating chocolate, proving it wasn't hypoglycemia, and I'd tried drinking extra water, proving it wasn't dehydration. That ruled out two of the possibilities he was going to suggest. If you feel much better after drinking more water, then you may have fixed the problem without the expense of a specialist appointment.